Heat treatment of light alloys



Patented Feb. 13, 1934.

UNITED STATES HEAT TREATMENT OF LIGHT ALLOYS Leopold Pessel,Philadelphia, Pa.

No Drawing.

2 Claims.

The object of this invention is to increase the resistance of heattreated light alloys against the intergranular embrittlement broughtabout by corrosion and to increase their elongation. I

have found that this can be accomplished by subjecting these alloys toconsiderable and prolonged static pressure after quenching but beforethe aging is completed, i. e. during the process of aging.

This invention relatesto the process of heat treating a certain classoflight alloys which are of such a nature that their tensile strengthcan be increased by heat treating; the latter will consist of thefollowing stages: (1) heating, (2) cooling or quenching, (3) aging. Theprocess of aging may take place at room temperature, at

higher or at lower temperatures, it may require a short or a long periodof time for its completion, but the period of aging must not be instantif measured at the lowest temperatures technically available. Withotherwords, the period of aging must be long enough, at suitabletemperatures, to be ascertained by customary physical tests at thebeginning and at the practical completion of the aging period.

The alloys falling into this class are principally duralumin and similaralloys, which generally contain not less than 90 percent aluminum, thebalance being composed of various elements, such as copper, magnesium,silicon, or others. There are, however, other light alloys showing theabove mentioned heat treatment characteristics. These alloys, whichgenerally contain not less than 75 per cent aluminum or magnesium orboth elements together, also fall within the claims of thisspecification.

Duralumin, which, among the alloys in question, shows the abovedescribed heat treating characteristics in a most pronounced manner, isvery extensively used, the most important field of application beingaircraft construction. A great disadvantage of this alloyis itssusceptibility to a form of corrosion known as intergranularembrittlement. Corrosion of this type will express itself in a decreaseof the tensile strength, but also, and in a much more pro-- nouncedmanner, in a decrease of the elongation. This intergranular corrosionmay take place while there is little indication of any changes on thesurface and yet the material may become so embrlttled that failure inservice may occur. Any method which decreases the susceptibility ofduralumin towards intergranular embrittlement or which raises itselongation without affecting the tensile strength promises to be ofusefulness to the aircraft industry.

While the main efforts to prevent intergranular corrosion have beenspent in the direction of applying protective coatings on the surface ofthe duralumin, they have not succeeded in render- Application February12, 3930 Serial No. 427,941

ing this material safe from the point of view of corrosion resistance.An important step was the discovery that duralumin quenched in coldwater and aged at room" temperature has a better resistance towardsintergranular corrosion than material that has been quenched in hotwater or aged at elevated temperatures. This has been, so far, the onlymethod of decreasing the susceptibility of duralumin towards this typeof corrosion by treatment of the alloy itself.

Intergranular corrosion is characterized by a loosening of the bondbetween the metal grains. In the case of duralumin it is believed thatthe hardening particles, especially those of the copper-alumiumcompound, have a tendency to gather along the crystal boundaries andthat some form of electrolytic action tends to weaken the bond betweenthe crystal grains. Any method that would diminish the number ofparticles along the grain boundaries or partly interfere with themigration of the particles from the interior of the grains towards theboundaries, would tend to produce a duralumin of superior resistanceagainst this type of corrosion. It would also have the efiect ofincreasing the elongation of the material.

By the method described in this specification the alloy is subjected toprolonged static pressure during the period of aging. It may be that theincreased rigidity of the compressed material at this stage of the heattreatment interferes with the movements of the particles, or that linesof strain are set up Within the grains which act as gathering points forthe particles and diminish the number of those actually reaching thegrain boundary. Whatever the theory of the causes of the phenomenon, Ihave found that duralumin, which has been kept under static pressureduring a considerable part of its aging period, has shown an increasedresistance against intergranular corrosion and an increased elongation.This method of improving corrosion resistance and elongation has-notbeen described before and is to be considered a new and originalinvention.

It is true that sometimes forming operations have been and are beingperformed on quenched duralumin before aging, such as pressing, bending,rolling etc., which may also involve a temporary application of kineticpressure.

The object of doing this work on the material immediately afterquenching (preferably within one hour), is to avoid the detrimentaleffects which cold working of the aged material hasupon the ductilityand corrosion resistance of the alloy. The purpose is to perform formingor straightening operations while the material is relatively soft. It iswell known that cold working of metals or alloys increases theirhardness and tensile strength and decreases their ductility. It alsotends to cause local strains which will be preferred centers ofcorrosion attack. It is just in order to avoid these effects that anymechanical operations that may be necessary are performed while thematerial is as soft as possible. Any kinetic pressure that may beapplied is only incidental and is released as soon as the required shapeor dimension is obtained.

In the method claimed by this specification the static pressure itselfis one essential factor. Its effect upon the metal grain is onlyincidental and is overshadowed by the effect upon the precipitation,growth, migration and distribution of the hardening particles. Theefiect of kinetic pressure upon the metal grain would be to decreaseelongation. With this method, however, the elongation may be increased.The element of time is the second important factor and cannot beinfluenced by mechanical means, as the above mentioned changes of theparticles require time, which can only be influenced by temperaturechanges. The time, during which the static pressure must be maintainedto influence the distribution of the particles, must be long enough topermit a certain growth.

As any kinetic pressure that may have been exerted upon duraluminincidental with its mechanical working before aging has always beenreleased after the required shape or dimension was obtained, any changeof such kinetic pressure into static pressure and prolongation of thelatter over a period of time reasonably suflicient to advance aging,would fall within the claims of this specification.

In duralumin the aging process makes very rapid progress immediatelyafter quenching, but becomes slower and slower with the time, withoutreaching a well defined end point. To express this fact properly, oneshould speak of the completion of aging when the material is practicallyaged. In duralumin of average composition, aging at room temperature,this practical limit is reached after approximately five days. To beeffective the static pressure must be applied before this state isreached. As a rule it will be desirable to apply the static pressure assoon as possible after quenching, before aging has progressed to anyconsiderable extent. The amount of static pressure and the length oftime during which it is to be exerted, will vary with the nature of thealloy, the temperature and the requirements regarding results. Thesefactors cannot generally be specified. It is well to remember that ifthe static pressure exceeds a certain limit a harmful effect upon themetal grain may be experienced. For practical purposes it may be assumedthat a beneficial effect will be observed if the static pressure is atleast 300 pounds per square inch and if the duration of the pressureperiod is not less than 10 minutes, provided the aging takes place atroom temperature. If the material is aged at artificially raisedtemperatures, the pressure period may be less than 10 minutes, but insuch a case it should be not less than 10 per cent of the aging period.

The static pressure may be transmitted to the surface of the material bymeans of solid, liquid or gaseous media. It may be convenient to usemechanical presses or to expose the material in suitable vessels tohighly compressed liquids or gases. Arrangements may be made to maintaina specified temperature during the application of pressure. Thetreatment of a commercial duralumin by this process and the results thathave been obtained are described below. It is obvious, however, that ananalogous treatment will give analogous results in that group of alloys,which, although of a different analysis, are commonly spoken of asalloys of the duralumin type. Furthermore, in view of the fact that theimprovement described in this specification consists in infiuencing'thecharacter and the distribution of precipitated hardening particles in analuminum matrix, analogous. results can be expected from hardeningparticles of a different chemical nature, as long as they have beendissolved in an aluminum matrix by a heating process, havebeenprecipitated by a. quenching operation to give greater strength to thematrix, and do not exert their full effect immediately but only in thecourse of a measurable time, commonly known as aging period.

The material consisted of duralumin sheet, inch thick, and of thefollowing approximate analysis: Cu 4.0%, Mg 0.5%, Si 0.35%, Mn 0.6%, Fe0.5%, the balance being aluminum. It was in a fully annealed conditionbefore heat treating. The heat treatment consisted in heating thematerial to 950 F. in an electric furnace, holding it at thistemperature for 20 minutes and quenching it in cold water. Immediatelyafter quenching it was transferred to ice water, to retard aging, andkept there for approximately 30 minutes. The material was then put underthe press plates of a press (screw type) and a pressure estimated atapproximately 600 pounds per square inch was applied. The material waskept under this pressure for 9 hours (at room temperature). It was thenremoved from the press and permitted to age completely at roomtemperature (requiring approximately one week) Some of the duraluminspecimens treated by this process were exposed to a salt spray corrosiontest lasting 30 days, together with other material that had the samecomposition and history, but had been aged without application or staticpressure. The following figures, which are average values obtained in anumber of determinations, show the relative physical qualities beforeand after exposure to corrosion, of duralumin treated according to thismethod and treated in the heretofore customary manner.

Tensile Strength (lbs/sq. inch) Before corrosion Alter corrosion Agedwith static pressure Aged without static pressure Elongation per cent Iin 2 inches Alter corrosion Before corrosion Aged with static pressureAged without static pressure I claim:

1. The method of increasing the elongation and resistance againstintergranular corrosion embrittlement of duralumin and alloys commonlyknown as alloys of the duralumin type, which are of such a nature thattheir strength can be increased by a heat treating process including aperiod of aging, which comprises subjecting these alloys, during theperiod of aging and for not less than nine hours, to a static pressurewhich will not result in any essential change of the shape or dimensionof the material and which shall be not less than 600 pounds per squareinch.

2. The method of increasing the elongation and resistance againstinter-granular corrosion embrittlement of duralumin and alloys commonlyknown as alloys of the duralumin type,

which are of such a nature that their strength can be increased by aheat treating process ineluding a periodof aging, which comprises agingthe material while subjecting it to a static pres sure which will notresult in any essential change of the shape or dimension of the materialand which shall be not less than 600 pounds per square inch.

LEOPOLD PESSEL.

